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LIX. Notices respecting New Boohs. 



The Magnetic Circuit. By Dr. H. du Bois. Translated by Dr. 

 Atkinson. Pp. xviii + 362. (London: Longmans, Green, & 

 Co. 1896.) 



TN view of the ever increasing employment of magnetic circuits, 

 -*■ in some form or other, in experimental and applied electricity, 

 the importance of this work (so far the only one in winch the 

 subject has been treated with anything like completeness) will be 

 recognized by both physicists and practical electricians. 



The book is divided into two parts and eleven chapters, the first 

 part (Chapters I.-Y.) being devoted to the mathematical develop- 

 ment of the theory, and the second chiefly to applications. 



Chapters I.-IV. contain a short account of the electromagnetic 

 field, the magnetization of bars and ellipsoids (considered as imper- 

 fect magnetic circuits), and the general theory of perfectly "rigid" 

 and of temporary magnetization. The mode of treatment of this 

 part in general follows that of Maxwell, but the definition of both 

 "magnetic intensity" and magnetic induction by the currents 

 induced in an exploring coil is an improvement on the usual 

 method in which the forces acting on a magnetic pole in narrow 

 cavities of different forms are considered at the outset. The term 

 "magnetic intensity" is thus used throughout the book (instead 

 of " magnetic force "), and magnetization is defined as the differ- 

 ence between magnetic induction and magnetic intensity, divided 

 by 4tt. 



In Chapter V. are explained the author's theory of the magneti- 

 zation of an iron toroid with a radial slit (the typical case of an 

 imperfect magnetic circuit), and the verification of the results by 

 the experiments of Lehmann. 



With Part II. begins the more special part of the work — the 

 properties of magnetic circuits as such. The discussion of the 

 stress existing in a circuit follows Maxwell in assuming a longi- 

 tudinal tension BH/47T— H 2 /8n- to exist at each point in the 

 direction of the lines of induction. Whether this is the magni- 

 tude of the stress existing (even in a rigidly magnetized part of 

 the circuit where also B and H have the same direction) is, of 

 course, open to question : the balance of evidence tends to show 

 that the stress has a different value. 



The remaining Chapters VII.-XI. are highly interesting, con- 

 taining a discussion of the analogy between the magnetic and other 

 circuits and the applicability of a so-called " Ohm's law," of the 

 magnetic circuits of dynamos and electromagnets and principles of 

 their design as realized in the du Bois electromagnet for producing 

 very strong fields, and, finally, of methods and instruments for 

 measuring field-intensity, magnetization, and induction. Of the 

 instruments described, one of the most interesting is the author's 

 magnetic balance, by means of which a complete magnetic cycle 

 (H, I) can be obtained in a few minutes. 



The translation, though good on the whole, is not altogether 



